Similarly to the Harlequin Duck, the only part of Europe where Barrow's Goldeneye is usually found is Iceland which I guess is why the binomial name includes islandica. I didn't see any there though, alas! Apart from that it is mainly found in North America. It can be told apart from the similar Common Goldeneye by the head shape - Barrow's is more of an elongated bizarre wedge-shape - and shape of the white mark behind the bill, which is more of a crescent in Barrow's and closer to a round spot in Common Goldeneye. It's a bit trickier with females though! Seems like the main thing to look out for is a bit more yellow on the bill, and a rounded rather than peaked crown.